1. Field
This disclosure relates generally to circuits and, more particularly, to level shifters in which both the high power supply voltage changes and the low power supply voltage changes.
2. Related Art
Level shifters play an important role in a variety of applications where power supply levels change. One way this has come up is in programming and erasing non-volatile memory cells. With the continuing reduction in power supply levels, the need for changing power supply levels with logic signals has become more prevalent. Integrated circuits commonly have more than one power supply level which has increased the need for level shifters. Most typically, the different voltage levels were positive voltages that were referenced to ground. There would be a voltage level VDD and another level VDL that was lower than VDD, but VDD and VDL were referenced to VSS which was normally ground. It is now become more common to not only vary the higher the voltage but also the lower voltage. Logic levels may be a logic low at a voltage VSL that is above VSS and a logic high at VDL that is below VDD. Different voltage levels for the both the logic high and the logic low make it more difficult to change from the level to another. One of the difficulties is in providing sufficient speed and another is in avoiding static power. When a level shifter is used to convert both the logic high and the logic low levels at the same time, a single stage delay may be preferred for high speed applications. After the level has been shifted, there should not be a steady state current path to ground. Reducing leakage current is becoming an increasingly important factor in extending battery life. It is thus desirable that any of transistors in the “non-conductive” state be strongly in that condition so as to reduce CMOS subthreshold leakage current. Another difficulty is in avoiding a multiple number of power supply rails used in a level shifter design when multiple logic levels are converted. Reducing the number of rails reduces routing congestion in the level shifter cell and at the boundary between two or more power supply voltage domains.
Accordingly, there is a need for a level shifter in which both the high power supply voltage changes and the low power supply voltage changes that overcomes or improves upon the problems described above or other problems.